ARTICLE pubs.acs.org/IC
Ammonium Octahydrotriborate (NH4B3H8): New Synthesis, Structure, and Hydrolytic Hydrogen Release Zhenguo Huang,† Xuenian Chen,†,‡ Teshome Yisgedu,† Edward A. Meyers,‡ Sheldon G. Shore,*,‡ and Ji-Cheng Zhao*,† †
Department of Materials Science and Engineering and ‡Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, United States
bS Supporting Information ABSTRACT: A metathesis reaction between unsolvated NaB3H8 and NH4Cl provides a simple and high-yield synthesis of NH4B3H8. Structure determination through X-ray single crystal diffraction analysis reveals weak NHδþ- - -HδB interaction in NH4B3H8 and strong NHδþ— HδB interaction in NH4B3H8 3 18-crown-6 3 THF adduct. Pyrolysis of NH4B3H8 leads to the formation of hydrogen gas with appreciable amounts of other volatile boranes below 160 °C. Hydrolysis experiments show that upon addition of catalysts, NH4B3H8 releases up to 7.5 materials wt % hydrogen.
’ INTRODUCTION The safe and efficient hydrogen storage is a major barrier to the use of hydrogen as a transportation fuel. Intensive efforts in recent years have been focused on boron hydrides such as metal borohydrides (Mg(BH4)2,1 NaBH42) and amine boranes (NH3B3H7,3 NH3BH34) for hydrogen storage because of their high hydrogen contents. Various catalysts have been investigated to improve hydrogen release from these materials through either thermal decomposition or hydrolysis. However, none of these materials under investigation meets the multiple targets set forth by the U.S. Department of Energy (DOE). The great variety and distinct properties of boron compounds prompt us to search for a compound that could satisfy the requirements for hydrogen storage. To obtain a high hydrogen capacity, it is essential to find a suitable combination of lightweight cation and H-rich BmHn anion. High number of m in BmHn leads to a lower hydrogen content and also greater stability of the boron cluster.5 Ammonium octahydrotriborate, NH4B3H8, has a high hydrogen content of 20.5 wt %. Unfortunately, the first synthesis reported 40 years ago involves pentaborane, B5H9, which is highly volatile and inflames violently upon contact with air.6a,b There are two pentaborane-free patents, but the procedures are complicated and require intermediate compounds and large amounts of solvents.6c,d Probably because of the lack of suitable syntheses, there have been little explorations of its reactivities and properties in 40 years. Here we report (1) a simple and efficient synthesis of this compound; (2) a r 2011 American Chemical Society
crystallographic study of the structure; (3) thermal decomposition; and (4) interesting hydrogen release properties through hydrolysis.
’ EXPERIMENTAL SECTION General Procedures. All manipulations were carried out on a high-vacuum line or in a glovebox filled with high purity nitrogen. Solvents were dried over sodium/benzophenone and freshly distilled prior to use. Ammonia (Matheson) was distilled from sodium immediately prior to use. NMR spectra were obtained on a Bruker Avance DPX 250 NMR. 11B NMR spectra were obtained at 80.3 MHz and externally referenced to BF3 3 OEt2 in C6D6 at 0.00 ppm. 1H NMR spectra were obtained at 250.1 MHz and referenced to THF-d8. Unsolvated NaB3H8 was prepared following the procedure recently developed in our lab.7 NH4Cl (Aldrich) was recrystallized from methanol. Mass spectra of the gas released from the hydrolysis and thermal decomposition were collected through a Balzer’s Quadstar 422 Quadrupole mass spectrometer. Synthesis of NH4B3H8. The prepared unsolvated NaB3H8 (634 mg, 10 mmol) reacted with NH4Cl (535 mg, 10 mmol) in liquid ammonia at 78 °C. After 30 min stirring using a magnetic bar, ammonia was removed and the flask was warmed to room temperature. The desired NH4B3H8 was extracted using dry tetrahydrofuran (THF) followed by pumping off THF right away, leaving a white powder, NH4B3H8 (555 Received: January 16, 2011 Published: March 15, 2011 3738
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Table 1. Crystallographic Information empirical formula formula weight
H12B3N 58.54
crystal system
orthorhombic
monoclinic
space group
Cmcm
Pc
Z a, Å
4 7.1530(14)
4 16.407(3)
b, Å
8.2648(17)
11.215(2)
c, Å
8.3895(17)
13.841(3)
R, deg
90
90
β, deg
90
107.76(3)
γ, deg
90
90
T, K
150
150
V, Å Dcalcd (g 3 cm3)
495.97(17) 0.784
2425.4(9) 1.082
μ (mm1)
0.038
0.079
λ, Å
0.71073
0.71073 8313
3
a
C16H44B3NO7 394.95
no. reflns collected
1617
no. unique reflns
251 (Rint = 0.0245)
4286 (Rint = 0.0258)
R1 [I > 2σ (I)]a
0.0370
0.0420
wR2(all data)b
0.1153
0.1043
R1 = ∑||Fo| |Fc||/∑|Fo|. b wR2=[∑w(F2oF2c )]2/∑w(F2o)2]1/2
mg, yield 95%). 11B NMR: (THF-d8) δ 29.9 ppm, 1H NMR (THFd8): NH4þ, δ 6.8 ppm; B3H8, δ 0.2 ppm. X-ray Crystallography. X-ray quality crystals of the NH4B3H8 3 18-crown-6 3 THF adduct were grown from 1:1 (molar ratio) mixtures of NH4B3H8 and 18-crown-6 in THF. NH4B3H8 single crystals were grown from anhydrous CH3CN. Caution! At room temperature, signs of decomposition (bubbles) were observed after a few days for both the NH4B3H8/18-crown-6/THF and NH4B3H8/CH3CN mixtures. X-ray single crystal diffraction data were collected on a Nonius Kappa CCD diffractometer which employs graphite-monochromated Mo KR radiation (λ = 0.71073 Å). Because of the sensitivity of the compound, single crystals were picked up from the mother solution in a nitrogen filled glovebox and stored in Fomblin oil until data collection. A single crystal coated with Fomblin oil was mounted on the tip of a glass fiber. Unit cell parameters were obtained by indexing the peaks in the first 10 frames and refined by employing the whole data set. All frames were integrated and corrected for Lorentz and polarization effects using the DENZO-SMN package.8 Absorption correction for the structure was accounted for by using SCALEPACK.8 The structure was solved (Table 1) by direct method using the SHELXTL-97 (full-matrix leastsquares refinements) structure solution package.9 All nonhydrogen atoms were located and refined anisotropically. Hydrogen atoms on boron and nitrogen atoms were located and refined isotropically, and other hydrogen atom positions were calculated by assuming standard geometries. Thermal Decomposition Studies. Thermal stability of NH4B3H8 was studied using a Mettler Toledo high-pressure differential scanning calorimeter (DSC27HP) in an argon glovebox with a ramp rate of 5 °C/min. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA 7 analyzer. Powder was loaded onto a quartz crucible and heated to 400 °C at a heating rate of 5 °C/min under an Ar flow of 40 cm3/min. The gaseous decomposition products were semiquantified through a calibrated vacuum line. After freezing out the gaseous products at 196 °C, hydrogen content was determined; after removing hydrogen and then raising the temperature to 78 °C, diborane was quantified; the residual product, pentaborane and borazine, were analyzed after diborane was removed and temperature was raised to ∼25 °C.10
Figure 1. 1H NMR spectrum of NH4B3H8 recorded in THF-d8 at room temperature. Inset is the 11B NMR spectrum.
Hydrolytic H2 Release Measurements. In a two-neck flask connected to a vacuum line, 1 mmol NH4B3H8 was mixed with 0.04 mmol CoCl2. After evacuating all the gases via dynamic vacuum, the flask was sealed and 18 mmol water was injected. After the reaction completion, the flask was opened to a vacuum line, and the evolved gas passed through a liquid nitrogen trap to isolate any non-hydrogen products, including water. The hydrogen was then quantitatively measured using a Toepler pump. About 8.80 mmol H2 was obtained through the reaction. For catalyst screening, a gas buret was set up to measure the amount of H2 gas and to determine the rates of H2 release.3a Pt/C (10 wt % Pt), Ru/Al2O3 (5 wt % Ru), RuCl3, CoCl2, NiCl2, and FeCl3 were all tested as received. For the catalyst loading, 1 mol % (with reference to NH4B3H8) of noble metal (Pt or Ru) was loaded, while 4 mol % of transition metal (Co, Ni or Fe) was employed.
’ RESULTS AND DISCUSSIONS A simple and efficient method has now been developed to synthesize NH4B3H8 via a metathesis reaction between unsolvated NaB3H8 and NH4Cl in liquid ammonia (eq 1). NH4 Cl þ NaB3 H8 f NH4 B3 H8 þ NaCl
ð1Þ
This reaction proceeds rapidly, and the desired product can be efficiently extracted using THF after removing the liquid ammonia. Furthermore, the yield is ∼95%, higher than 79.5% reported for the previous methods.6 The unsolvated NaB3H8 was prepared via a route recently developed in our lab,7 which eliminates the use of dangerous starting materials such as B2H6 and BF311 and enables a high yield. Therefore, a large-scale and simple synthesis of NH4B3H8 is now possible. The metathesis reaction has also been explored in several other solvents, and it was found that the solvent had an influence on the course of the reaction. In addition to NH4B3H8, NH3B3H7 and other boron compounds were also observed for the reaction in THF and DME, with the latter resulting in more NH3B3H7. The successful synthesis in liquid ammonia is corroborated by the NMR spectrum (Figure 1). After extracting NH4B3H8 using THF and then pumping off the THF immediately via dynamic vacuum, pure desired product was obtained. NH4B3H8 exhibits a 11 B NMR resonance at 29.9 ppm, which is assigned to B3H8.7 The 1H NMR spectrum of NH4B3H8 reveals the existence of both NH4þ (6.8 ppm) and B3H8 (0.2 ppm). The structures of NH4B3H8 and NH4B3H8 3 18-crown6 3 THF adduct were determined through single crystal X-ray diffraction analysis. Selected bond distances and angles are given in Tables 2 and 3, respectively. Figure 2 shows a separate ion pair structure of NH4B3H8, with the shortest NHδþ---HδB distance being 2.37(3) Å, very close to the sum of van der Waals 3739
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Table 2. Selected Bond Distances and Angles of H12B3Na N(1)H(5)
0.85(3)
N(1)H(6)
0.79(4)
B(1)B(2)
1.779(2)
B(1)B(2)#
1.779(2)
B(1)H(4) B(1)H(1)
1.475(18) 1.079(17)
B(2)B(2)#
1.813(3)
B(2)H(3)
1.096(13)
B(2)H(4)
1.137(17)
H(5)N(1)H(6)
113(2)
B(2)B(1)B(2)#
61.27(11)
B(2)B(1)H(4)
39.5(7)
B(2)#B(1)H(4) B(2)B(1)H(1)
100.8(7) 117.3(7)
B(2)#B(1)H(1)
117.3(7)
H(4)B(1)H(1)
100.4(4)
B(1)B(2)B(2)#
59.37(5)
B(1)B(2)H(3)
121.5(6)
B(2)#B(2)H(3)
108.5(7)
B(1)B(2)H(4)
55.7(9)
B(2)#B(2)H(4) H(3)B(2)H(4)
115.0(9) 104.2(8)
Figure 2. Crystal structure of NH4B3H8. B, pink; H, light gray; N, blue.
a Symmetry transformations used to generate equivalent atoms. # = x, y, z þ 3/2.
Table 3. Selected Bond Distances and Angles of C16H44B3NO7 N(1)H(2)
0.82(5)
N(1)H(1)
0.92(4)
N(1)H(3)
0.87(4)
N(1)H(4)
0.99(6)
B(1)B(3)
1.758(6)
B(1)B(2) B(2)B(3)
1.796(7) 1.762(6)
B(1)H(9)
1.14(4)
B(1)H(15)
1.27(5)
B(2)H(11)
1.10(4)
B(2)H(12)
1.08(5)
B(3)H(16)
1.41(4)
H(2)N(1)H(3)
111(4)
H(1)N(1)H(3) H(2)N(1)H(4)
108(3) 111(4)
H(1)N(1)H(4)
110(4)
B(3)B(1)B(2)
59.4(3)
B(1)B(3)B(2)
61.3(3)
B(3)B(2)B(1)
59.2(2)
B(3)B(1)H(9)
121.4(18)
B(3)B(1)H(10)
126(2)
B(2)B(1)H(15) B(1)B(3)H(16)
115(2) 102.0(17)
B(2)B(3)H(16)
40.6(17)
B(1)B(3)H(15)
45.1(19)
B(1)B(3)H(14)
115.2(18)
radii between two hydrogen atoms, 2.4 Å. All other NHδþ--HδB distances are more than 2.47 Å. This seems unusual
Figure 3. Fragment of the NH4B3H8 3 18-crown-6 3 THF adduct. B, pink; H, light gray; N, blue; O, red; C, gray.
since BN compounds tend to have close NHδþ---HδB contacts,12a as observed in NH3B3H73a and NH3BH3.12b This structure differs from the reported metal octahydrotriborates, where B3H8 can coordinate with a metal center such as for Cr(B3H8)2,11c (CO)4Mn(B3H8),13 Mg(B3H8)2(Et2O)2,14 and Ru(B3H8)(PPh3){κ3-HB(pz)3}.15 The molecular structure of NH4B3H8 3 18-crown-6 3 THF adduct is illustrated in Figure 3. Ammonium cation has been reported to connect to crown ether by three NH 3 3 3 O hydrogen bonds between the hydrogen atoms of the ammonium and the oxygen atoms of the crown ether.16 We likewise found similar interaction present in NH4B3H8 3 18-crown-6 3 THF adduct. There is another independent fragment in the asymmetric unit, very similar to the one shown in Figure 3. The THF molecule is packed into the structure with no interactions with other moieties. The average NO distance is 2.94 Å, longer than the standard NO hydrogen bond length (2.87 Å).17 The hydrogen atom on nitrogen that is not involved in bonding to an oxygen atom interacts with three terminal hydrogen atoms on the boron triangle, with the closest NHδþ---HδB distance of 1.92(5) Å being shorter than 2.13(3) Å present in NH3B3H7 determined by X-ray diffraction analysis,3a significantly less than the sum of van der Waals radii between two hydrogen atoms, 2.4 Å. NH4B3H8 in pure and solid form is stable at room temperature with no detectable signs of decomposition within one month. On the contrary, the adduct decomposes slowly at room temperature with the evolution of hydrogen gas. The poor stability of the adduct is likely due to the stronger NHδþ---HδB interactions. 3740
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Figure 5. 11B NMR spectra of 2 M aqueous solutions taken after (a) 1 day for NaBH4; (b and c) 1 day and 7 days for NH4B3H8. Inset is an expanded spectrum of c. *, B5O6(OH)4; †, B3O3(OH)h4 and B5O6(OH)h. 4
Figure 4. DSC and TGA curves of NH4B3H8 (5 °C/min).
NH4BH4, an analogue of NH4B3H8, decomposes at 78 °C in THF.18 This is in stark contrast to the slow degradation of NH4B3H8 in THF at ambient conditions. Since both compounds contain the same cation, the difference is likely due to the weaker hydridic Hδ of B3Hh8 compared to that of BH4. This is consistent with the findings that for boron hydrides the higher number of boron atoms in the structure, the less hydridic the hydrogen is.19 NH4B3H8 has a high hydrogen content, 20.5 wt %. Unfortunately, thermal decomposition analysis shows that this compound gives off 10 wt % hydrogen below 160 °C, but with appreciable amounts of diborane, pentaborane, and borazine (Supporting Information). DSC and TGA results (Figure 4) collectively indicate that melting and decomposition take place simultaneously at 120 °C, with a substantial weight loss at this temperature. The presence of the boranes results in a ∼70% final weight loss. It has been reported that NH3BH3 gives off the first equivalent H2 below 120 °C and further releases hydrogen at higher temperature but with a small fraction of borazine.20 Compared with NH3BH3, NH4B3H8 produces less hydrogen and more undesired volatile compounds below 160 °C. Thus, NH4B3H8 is probably not a suitable candidate for hydrogen storage through thermal decomposition. Hydrolytic studies, however, have demonstrated that NH4B3H8 has advantages over NaBH42 and NH3BH3,4c,d the most studied chemical hydrides for hydrogen storage via hydrolysis. At room temperature, NH4B3H8 is more soluble in water (>45 wt % through visual observation) than NH3BH3 (26 wt %21) and NaBH4 (35 wt %22). The lower solubility significantly limits the theoretical hydrogen density of the respective system to 5.1 wt % for NH3BH3 and 7.5 wt % for NaBH4. Quantitative measurements of H2 released via hydrolysis using a Toepler pump and gas buret showed that upon adding catalysts, 1 mmol NH4B3H8 releases a near theoretical value of 8.80 mmol H2. Based on eq 2, therefore, this system has a high theoretical hydrogen weight density [wt % = H2/(NH4B3H8 þ H2O)] of 10.8 wt %. cat:
NH4 B3 H8 ðsÞ þ 6H2 OðlÞ sf NHþ 4 ðaqÞ þ 3BO2 ðaqÞ
þ 2Hþ ðaqÞ þ 9H2 ðgÞ
ð2Þ
In addition, unlike NaBH4, which is stable only in strong alkaline solutions,2 an aqueous NH4B3H8 solution is reasonably stable as shown by 11B NMR studies (Figure 5). At 28 °C, for the same molar concentration, that is, 2 M, 11B NMR studies show that more than 50% of NaBH4 decomposed over a day, while less than 10% of NH4B3H8 decomposed over a week. Hydrolytic studies have shown that upon adding catalysts NH4B3H8 rapidly releases pure H2 (Figure 6). For an aqueous
Figure 6. Hydrogen evolution at room temperature from an aqueous NH4B3H8 solution (molar ratio of NH4B3H8:H2O being 1:18) containing 10 wt % Pt/C (1 mol % Pt); 5 wt % Ru/Al2O3 (1 mol % Ru), RuCl3 (1 mol % Ru), CoCl2 (4 mol % Co), NiCl2 (4 mol % Ni), and FeCl3 (4 mol % Fe). Molar ratios are referenced to NH4B3H8.
solution with a 1:18 molar ratio of NH4B3H8 to H2O, which represents a material density of 4.6 wt % H, the commercially available 10 wt % Pt/C catalyst (1 mol % Pt) shows the best catalytic activity, with complete hydrolysis in less than 30 min. RuCl3 also displayed excellent catalytic activity, and this is likely associated with the in situ formation of Ru(0) as indicated by the formation of a black powder, which was also observed during hydrolysis of both NaBH42 and NH3BH3.4c However, 5 wt % Ru/Al2O3 (1 mol % Ru) is much less active. The difference is probably associated with the degree of the dispersed catalytic sites. Lower-cost transiton metal catalysts were also explored in our study, and fairly good catalytic activity was observed for CoCl2. With 4 mol % loading of CoCl2, full hydrolysis was complete in about 100 min. A black powder appeared instantly when CoCl2 was brought into contact with an aqueous NH4B3H8 solution, suggesting the formation of cobalt boride (Supporting Information) as found during the hydrolysis of NaBH4.2 NiCl2 is less effective and the full hydrolysis requried ∼200 min. The “BO2” product shown in eq 2 is only a hypothetical formulation, with the actual borate products of these reactions depending upon the final solution concentration and pH.3a The 11B NMR spectrum (Figure 7) of the catalyzed hydrolysis solution of NH4B3H8 and H2O (1: 18 ratio) reveals one broad peak centered at 16 ppm which is likely associated with B3O3(OH)4 and B5O6(OH)4, along with another sharp peak situated at around 0 ppm that is also due to B5O6(OH)4.23 Similar products were also formed during the hydrolysis of NH3B3H7.3a 3741
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Figure 7. 11B NMR spectrum of the catalyzed hydrolysis solution (NH4B3H8: H2O being 1: 18).
When an aqueous solution with 1:10 molar ratio of NH4B3H8 to H2O was subject to hydrolysis catalyzed by 4 mol % of CoCl2, full hydrolysis was achieved in ∼200 min, which corresponds to a production of 7.5 wt % H. The U.S. DOE recently modified the vehicular hydrogen storage targets to 4.5 wt % by 2010, 5.5 wt % by 2015, and 7.5 wt % for the ultimate.24 NH4B3H8 thus could be a potential candidate to meet the 2010 target. Calculations of the standard heat of hydrolysis from the standard enthalpies of formation indicate that H2 release from NH4B3H8 is much less exothermic (5.43 kcal/mol H2, Supporting Information) than from either NaBH4 (14.9 kcal/mol H2), NH3BH3 (12.7 kcal/mol H2), or NH3B3H7 (18.9 kcal/mol H2).3a The less exothermic nature favors NH4B3H8 over the other candidates from the standpoint of heat management and system design. As in the cases of NaBH4, NH3BH3, and NH3B3H7, the formation of borates may affect the performance of the catalysts if the borate precipitates cover the active sites. It should be noted that the removal and transportability of the borates depend on the starting materials and reaction conditions,25 and thus can be improved accordingly. Regeneration of these compounds from the borates has been challenging, but progress has been made in the regeneration of spent fuel back into NH3BH3.26 The final utility of NH4B3H8 as a chemical hydrogen storage material will also hinge on the development of an efficient “off-board” regeneration of NH4B3H8.
’ ASSOCIATED CONTENT
bS
Supporting Information. Structural information, calculation of the heat of the hydrolysis, mass spectra of gas from hydrolysis and thermal decomposition. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected] (J.-C.Z.),
[email protected] (S.G.S.).
’ ACKNOWLEDGMENT This work was funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) under Contract No. DE-FC3605GO15062. ’ REFERENCES (1) (a) Soloveichik, L. G.; Gao, Y.; Rijssenbeek, J.; Andrus, M.; Kniajanski, S.; Bowman, R. C.; Hwang, S.-J.; Zhao, J.-C. Int. J. Hydrogen Energy 2009, 34, 916. (b) Matsunaga, T.; Buchter, F.; Miwa, K.; Towata, S.; Orimo, S.; Z€uttel, A. Renewable Energy 2008, 33, 193.
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